The present invention relates to an electrochemical capacitor used in a variety of electronic devices.
An electric double layer capacitor characterized by having a capacitance value between that of a battery and that of a capacitor has been widely used as a DC power source of a small wattage rating to replace a back-up power supply or to serve as an auxilary/alternate cell of a secondary battery for ICs and memories. However, the energy density of electric double layer capacitors available at present is some tenth of the energy density of such secondary batteries as a lead acid storage battery, a nickel hydrogen secondary battery and therefore a much higher energy density has been required of electric double layer capacitors.
On the other hand, the advantages in using an electric double layer capacitor are as follows:
Such environmentally burdening material as lead, cadmium and the like are used in the electrodes of general purpose secondary batteries, whereas electric double layer capacitors use electrodes mainly formed of such carbon materials as activated carbon and the like that are safe and less burdensome on the environment. In addition, electric double layer capacitors utilize electric double layer capacitance created on the interface between an electrode and an electrolyte, not relying on the conversion of a chemical reaction to electric energy like in a storage battery, resulting in a high degree of efficiency at the time of charging/discharging and also a long cycle life.
An electric double layer capacitor comprises a pair of polarizable electrodes, each of which is formed by the steps of applying a slurry comprised of activated carbon, carbon black, a binder and the like onto an electric current collector formed of an aluminum foil and the like to a uniform thickness and finally drying the slurry, disposed opposite to each other with a porous separator, which is impregnated with a corrosion-resistant electrolyte, sandwiched between the polarizable electrodes.
A water-soluble type electrolyte or an organic solvent type electrolyte (a nonaqueous electrolyte) is used as the electrolyte for electric double layer capacitors. However, the withstand voltage of a water-soluble type electrolyte is low (about 1.2 V), resulting in a problem of difficulties to realize electric double layer capacitors with a high energy density. On the other hand, the withstand voltage of a nonaqueous electrolyte ranges from 2.3 V to 3.0 V but the electric conductivity thereof is low when compared with a water-soluble type electrolyte, resulting in a problem of increasing the internal resistance of electric double layer capacitors. In recent years, various studies have been carried out to solve these problems and such inventions as disclosed in the Japanese Patent Application Unexamined Publication Nos. S61-32509 and S62-237715, the International Patent Application Unexamined Publication No. WO96/20504 and the like have been made.
The activated carbon to form electrodes for electric double layer capacitors is prepared by carbonizing at high temperatures such raw materials as coconut husk, sawdust, phenolic resin, petroleum coke, coal coke and the like and thereafter by putting through such treatments as steam-activation, zinc chloride activation, alkali activation and the like. It is generally said that the surface area per unit weight of the activated carbon for electric double layer capacitors is the larger, the higher capacitance per unit weight is allowed to be gained. However, an increase in the surface area per unit weight of the activated carbon results in a decrease in the bulk density thereof, thereby causing a problem of decreasing the energy density per unit volume of electric double layer capacitors.
Further, in order to solve the foregoing problems associated with electric double layer capacitors, such new devices, which have the energy density comparable with that of secondary batteries such as a lead acid storage battery and the like, as an electrochemical capacitor as described in the Japanese Patent Application Unexamined Publication No. H6-104141 and the like have been recently developed and many reports cover such developments. In addition, a xcfx80 conjugate polymer as described in Chem. Rev. 1997, 97, 207-281 and studied for use in the electrode of secondary batteries is also allowed to be used in an electrochemical capacitor theoretically.
However, even among the foregoing conventional electrochemical capacitors, an electrochemical capacitor with an electrode formed of ruthenium oxide or a composite of ruthenium oxide and other metal oxides realizes a high energy density and excellent cycle characteristics on one hand but on the other hand there is such a problem as the ruthenium used in the electrode being very expensive and the like. An electrochemical capacitor with such conductive polymers having a xcfx80 conjugate as polypyrrole, polythiophene, polyaniline and the derivatives thereof and the like used as the electrode materials is allowed to realize a high energy density but also has a problem of being inferior to the electric double layer capacitors utilizing activated carbon in terms of heat resistance and cycle characteristics. This is attributed to the heat resistance of a xcfx80 conjugate type conductive polymer and an irreversible chemical reaction occurring between the electrolyte material needed for an electrochemical capacitor and the xcfx80 conjugate type conductive polymer. Therefore, an electrode material, which has high heat resistance and does not cause an irreversible chemical reaction with the electrolyte material, is being looked for. Furthermore, it is generally difficult for a xcfx80 conjugate conductive polymer to be resolved/melted, resulting in a problem of not allowing the electrode for electrochemical capacitors to be produced readily.
The present invention deals with the foregoing problems that have been so far existing and aims at providing an electrochemical capacitor having a high energy density and excelling in heat resistance and charge/discharge cycle characteristics.
In order to solve the foregoing problems, the present invention discloses an electrochemical capacitor comprising a pair of oxidation-reduction active electrodes and an electrolyte or an ionic conductive solid electrolyte sandwiched between the oxidation-reduction active electrodes, in which the oxidation-reduction active electrode contains a copolymer that has a chemical formula 1 as described below, thereby allowing an electrochemical capacitor having a high energy density and excelling in heat resistance and charge/discharge cycle characteristics to be realized.
Raxe2x80x94(C2Rc4)lxe2x80x94(OC2Rd4)mxe2x80x94(X)nxe2x80x94Rb, or
Raxe2x80x94[xe2x80x94(C2Rc4)lxe2x80x94(OC2Rd4)mxe2x80x94(X)nxe2x80x94]pxe2x80x94Rbxe2x80x83xe2x80x83(Chemical Formula 1)
, where Ra and Rb are, respectively, hydrogen, fluorine, an alkyl group with 1 to 15 of carbon, a fluoroalkyl group, an alkenyl group and a fluoroalkenyl group, or a phenyl group, a fluorophenyl group, a sulfone group, an amino group, a nitro group and a cyano group, and Ra and Rb are allowed to be the same with or different from each other.
Also, Rc and Rd are selected from hydrogen, fluorine, an alkyl group with 1 to 15 of carbon, a fluoroalkyl group, an alkenyl group and a fluoroalkenyl group, or a phenyl group, a fluorophenyl group, a sulfone group, an amino group, a nitro group and cyano group, or a chemical formula A, and the respective four Rc and Rd are allowed to be the same with or different from one another.
The chemical formula A is described by a general expression as follows:
xe2x80x94(CO)ORe, xe2x80x94(CH2)Raxe2x80x94(OC2Rb4)bxe2x80x94ORe,
where Re is hydrogen or an alkyl group, and xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are integers of 0 to 20 and allowed to be the same with or different from each other.
In the chemical formula 1, xe2x80x9clxe2x80x9d and xe2x80x9cmxe2x80x9d are integers of 0 or larger and not 0 at the same time, xe2x80x9cnxe2x80x9d and xe2x80x9cpxe2x80x9d are natural numbers of 1 or larger. X is any one species, or two or more species out of the chemical species described by a general expression as follows: 
R1 to R6 are, respectively, hydrogen, fluorine, an alkyl group with 1 to 20 of carbon, which is allowed to be replaced with a hydroxyl group, a fluoroalkyl group, an alkenyl group and a fluoroalkenyl group, or a phenyl group, a fluorophenyl group, a sulfon group, an amino group, a nitro group and a cyano group with part or all thereof being allowed to be linked with one another to form rings. Also, R1 to R6 are allowed to be the same with or different from one another.